1. Field of the Invention
The present invention relates to a technology for formation of toner patches used to control toner density in image formation in an image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus, such as a copier or a laser beam printer, an image-density control process is performed in the following manner to ensure a stable image density. That is, a tone pattern formed of 10 to 17 toner patches is formed on an image carrier, such as a photosensitive element, under different image forming conditions (with different developing potentials) so that they the toner patches have different toner adhesion amounts (i.e., toner densities), respectively. The toner adhesion amount of each of the toner patches is calculated based on a detection value of the toner patch optically detected by an optical sensor, and a predetermined algorithm set for calculating the toner adhesion amount. Then, an equation of a straight line, that is, y=ax+b is obtained based on a relationship between the toner adhesion amount of each of the toner patches and the image forming condition (the developing potential), and then a development index value γ that is an index of development performance (i.e., the slope “a” of the graph of y=ax+b, with a developing potential on a horizontal axis and a toner adhesion amount on a vertical axis) and a development start voltage Vk (i.e., the intercept “b” of the graph of y=ax+b) are obtained. Then, image forming conditions such as a laser diode (LD) power, a charging bias, and a developing bias are adjusted based on the development index value γ and the development start voltage Vk so that a developing potential corresponding to a proper toner adhesion amount can be obtained.
The optical sensor used for detecting the toner patches includes a light emitting element, such as a light emitting diode (LED), and a light receiving element, such as a phototransistor. The light emitting element irradiates the toner patch with a light and the light receiving element detects an amount of reflected light from the toner patch. Generally, the optical sensor can sensitively detect a toner patch having a small toner adhesion amount, but it cannot sensitively detect a toner patch having a larger toner adhesion amount than a predetermined toner adhesion amount depending on a detection sensitivity of the light receiving element. In other words, the optical sensor has a predetermined detection range in which the toner patches can be detected sensitively. Therefore, to accurately obtain the development index value γ and the development start voltage Vk, the toner adhesion amounts of the toner patches of the tone pattern need to be distributed evenly from a small toner adhesion amount to a large toner adhesion amount within the detection range of the optical sensor in which the toner patches can be detected sensitively.
In the conventional technology, to accurately calculate the development index value γ and the development start voltage Vk even when the development index value γ increases or decreases, 10 to 17 toner patches having different toner adhesion amounts are formed with different fixed developing biases, respectively. The reason for this is as follows. When the development index value γ increases, the development performance also increases, so that an image having a relatively large toner adhesion amount can be formed with a low developing bias. Accordingly, even when the toner patch is formed with a developing bias of a middle level, the toner adhesion amount thereof may be out of the detection range of the optical sensor. Therefore, to ensure the condition in which the toner adhesion amounts of the toner patches of the tone pattern are distributed evenly from a small toner adhesion amount to a large toner adhesion amount within the detection range of the optical sensor even when the development index value γ increases, it is necessary to form a plurality of toner patches with low developing biases.
On the other hand, when the development index value γ decreases, the development performance also decreases, so that an image having a relatively large toner adhesion amount can be formed only with a high developing bias. Therefore, if all the toner patches of the tone pattern are formed with low developing biases, and when the development index value γ decreases, even the largest toner adhesion amount of the toner patches remains in a small toner adhesion amount range. As a result, the toner adhesion amounts of all the toner patches are concentrated in the small toner adhesion amount range. If the toner adhesion amounts of all the toner patches are concentrated in the small toner adhesion amount range, the development index value γ and the development start voltage Vk cannot be calculated accurately because of effects of fluctuation in the toner adhesion amounts. Therefore, to accurately calculate the development index value γ and the development start voltage Vk even when the development index value γ decreases, a plurality of toner patches formed with high developing biases need to be prepared in addition to the toner patches formed with low developing biases.
In this manner, to accurately calculate the development index value γ and the development start voltage Vk even when the development index value γ increases or decreases, the toner patches need to be formed with both low developing biases and high developing biases. Therefore, in the conventional technology, at least 10 to 17 toner patches need to be formed in one tone pattern because of necessity of forming the toner patches both with low developing biases and high developing biases. However, as the number of the toner patches increases, a processing time for adjusting an image density increases and toner consumption also increases. Particularly, in a color image forming apparatus that forms an image by using four color toners Y, M, C, and K, because the tone pattern is formed for each color, the processing time for adjusting the image density more increases.
Japanese Patent Application Laid-open No. 2006-106222 discloses an image forming apparatus that performs an image-density control process in the following manner. That is, the development index value γ and the development start voltage Vk calculated based on a detection result obtained by the optical sensor are stored in a storage unit. Then, in a next image-density control process, developing biases used for forming toner patches are calculated, respectively, based on the development index value γ and the development start voltage Vk stored in the storage unit so that the toner adhesion amounts of the toner patches can be evenly distributed from the small toner adhesion amount to the large toner adhesion amount within the detection range of the optical sensor. Then, the tone pattern is formed with the calculated developing biases, and the image-density control process is performed by using the tone pattern. Generally, the development index value γ does not largely fluctuate from the previous development index value γ. Therefore, if the previous development index value γ is used when forming the toner patches, it is possible to evenly distribute the toner adhesion amounts of the toner patches from the small toner adhesion amount to the large toner adhesion amount within the detection range of the optical sensor with a small number of the toner patches. More particularly, when the development index value γ is large, all toner patches are formed with low developing biases. When the development index value γ is small, toner patches having toner adhesion amounts in a small toner adhesion amount range are formed with low developing biases while toner patches having toner adhesion amounts in a large toner adhesion amount range are formed with high developing biases. In this manner, if the developing biases for the toner patches are determined based on the previous development index value γ, the toner adhesion amounts can be evenly distributed from the small toner adhesion amount to the large toner adhesion amount within the detection range of the optical sensor with a less number of the toner patches than that in the conventional technology in which all the toner patches are formed by using the fixed developing biases, even when the development index value γ decreases or increases. Thus, the number of the toner patches can be reduced. As a result, a processing time for the image-density control process can be reduced and a consumption amount of toner by the image-density control process can also be reduced.
However, in some cases, e.g., when an image-density control process is performed after an environmental condition of an image forming apparatus largely changes or the image forming apparatus is not in use for a long time, the development index value γ may largely increases from the previous development index value γ.
If this happens, the following problem may occur in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-106222. That is, among the toner patches formed with the developing biases calculated based on the previous development index value γ, only one toner patch having the smallest toner adhesion amount is within the detection range of the optical sensor, which makes calculation of the development index value γ difficult. In this case, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-106222, the tone pattern is formed again by changing a developing bias or increasing the number of the toner patches contained in the tone pattern so that at least two different toner adhesion amounts of the toner patches can be within the detection range of the optical sensor.
However, because the tone pattern is formed again, a processing time for the image-density control process increases, increasing a downtime of the image forming apparatus. Furthermore, the toner consumption also increases.
The applicant of the present invention has proposed an image forming apparatus in Japanese Patent Application No. 2008-28139. In the image forming apparatus, some toner patches among toner patches of a tone pattern are formed under a predetermined fixed image forming condition (hereinafter, theses toner patches are referred to as “fixed toner patches” as appropriate). The rest of the toner patches are formed under an image forming condition determined based on a previous image forming condition that is adjusted in a previous process (hereinafter, these toner patches are referred to as “variable toner patches” as appropriate). The predetermined fixed image forming condition is determined such that the toner adhesion amounts can be within the detection range of the optical sensor even when the development index value γ increases.
Accordingly, when the current development index value γ does not greatly vary from the previous development index value γ used for adjusting the previous image forming condition, the toner adhesion amounts of the variable toner patches can be evenly distributed within the detection range of the optical sensor. Therefore, the development index value γ can be accurately calculated even with a small number of the toner patches. As a result, the image forming condition can be accurately adjusted.
On the other hand, when the current development index value γ largely increases from the previous development index value γ, the toner adhesion amounts of the fixed toner patches can be remained within the detection range of the optical sensor. Therefore, when the current development index value γ largely increases from the previous development index value γ, and even if only one variable toner patch has a toner adhesion amount within the detection range of the optical sensor, the toner adhesion amounts of at least two toner patches can be within the detection range of the optical sensor. Therefore, the development index value γ can be obtained and the image forming conditions can be adjusted. In other words, by performing only a single process of forming the tone pattern, the image forming condition can be adjusted. Thus, because the tone pattern need not be formed again, increase in the processing time for the image-density control process, increase in downtime of the image forming apparatus, and increase in the consumption amount of toner by the image-density control process can be prevented.
However, the following problem remains in the image forming apparatus proposed in the Japanese Patent Application No. 2008-28139. That is, in the color image forming apparatus, when the current development index value γ largely increases or largely decreases from the previous development index value γ, the calculation accuracy of the development index value γ for a certain color decreases compared to the other colors, so that the image-density control process as a whole cannot be performed accurately.